Re: how and what with to polishing silver sax....
Honestly there's no easy way to clean it. DO NOT ATTEMPT TO USE LIQUID POLISH ON AN ASSEMBLED HORN! You will surely get polish in places it's impossible to access, and there's an excellent chance some of the polish will get onto the steel pivot screws and rods...corrosion and unhappiness will surely result.
If the tarnish is minor buy a silver polishing cloth...available at most jewelers, or places like Bed, Bath and Beyond. To get under keys you'll need to cut the cloth into 1" strips and pull it back and forth under the springs and around the posts. There's no difference between the brushed silver and the bright.
If the horn is badly tarnished, the proper method is to remove all the keys. If you dip the body in a silver cleaning solution you'll need to remove all the springs...the solution will react with the steel and corrode the springs.
If you remove the keys, be prepared to be stuck by the springs at least 6-8 times before you're done!
I hope this helps. Feel free to contact me directly with any additional questions: firstname.lastname@example.org, www.woodwindshop.com